75/140 would be the best to stick with. 75/110 is going to be thinner than 75/140, not heavier. Thinner may give you slightly less resistance, ala slightly better mileage, but at the cost of increased wear. Also if you have a limited slip diff, don't forget the additive for that.

I'd go for the 75-110 instead of the 75-140, or else get something with a higher cold weight if you want the 140 hot weight. Oils that aren't straight-weight (i.e. the ones with two weight numbers) have chemicals called pour-point depressants mixed in, and they cause the oil to behave like it's a lighter weight than it really is when it's cold.

That sounds great, and under normal use it isn't a problem, but those pour-point depressants wear out with age and start affecting the oil's weight when it's hot too -- so the 75-140 will actually be more like a 60-90 after a couple dozen thousand miles, whereas the 75-110 might be more like 70-100.

Rule of thumb: the smaller the weight spread, the more the oil thins from heat, and the wider the weight spread, the more the oil thins from age. Since gear oil doesn't get changed very often, I think it's better to pick something that will retain its viscosity for a long time.

EDIT: I did not google this, I asked several mechanics and a tribologist about it.

my suggestion: Royal Purple 75-140 .... its the best shit i have ever used in any diff / manual transmission ....

stick with factory weight, and dont listen to DE, he is retarded.

texas gets hotter than it does colder ... stay with thicker oil. when you move to canada, consider thinner oil, otherwise, the SE/Souther US does not get cold enough to consider thinner oils.

fuck man, i run thicker than spec'd diff / manual trans / transfer case oil ... and i have NEVER had a wear related failure in any of them.

you have larger tires, and a lift, and i assume (or hope) you at least take that POS offroad .... you are putting more stress and more wear on the gears than normal highway usage ... the LAST thing you need is thinner oil.

I'd go for the 75-110 instead of the 75-140, or else get something with a higher cold weight if you want the 140 hot weight. Oils that aren't straight-weight (i.e. the ones with two weight numbers) have chemicals called pour-point depressants mixed in, and they cause the oil to behave like it's a lighter weight than it really is when it's cold.

That sounds great, and under normal use it isn't a problem, but those pour-point depressants wear out with age and start affecting the oil's weight when it's hot too -- so the 75-140 will actually be more like a 60-90 after a couple dozen thousand miles, whereas the 75-110 might be more like 70-100.

Rule of thumb: the smaller the weight spread, the more the oil thins from heat, and the wider the weight spread, the more the oil thins from age. Since gear oil doesn't get changed very often, I think it's better to pick something that will retain its viscosity for a long time.

EDIT: I did not google this, I asked several mechanics and a tribologist about it.

Click to expand...

and you wonder why everyone hates you.

stick with stock weight. id rec. normal dino oil too. lots of gear manufacturers wont warrenty gears that have been run with synthetic oil since they "sharpen" the gears and cause a whining noise.